We have demonstrated that there is a gradual increase in the responsiveness of the hypothalamic-pituitary system responsible for gonadotropin release to testosterone when castrated golden hamsters are transferred from a stimulatory to a nonstimulatory photoperiod. The pineal gland is involved in this photic-induced change in steroid feedback sensitivity since pinealectomy abolishes this response. Experiments are in progress which are designed to determine: 1) How the pineal gland alters the negative feedback effects of testosterone on pituitary gonadotropin release, 2) If prolactin is involved in this response and 3) Whether there is a similar change in sensitivity to other steroid hormones. Our experiments over the past two years have demonstrated that both the pineal gland and steroid hormones are involved in photic-induced changes neuroendocrine gonadal function in hamsters. Due to the central role that the circadian system plays in photoperiodic time measurement in vertebrates, we are beginning to examine the relationship between the pineal gland, steroid hormones and the biological clock in an attempt to determine if such a clock system mediates the effect of pineal and gonadal hormones on hypothalamic-hypophyseal activity. Our preliminary results indicate that pinealectomy, as well as the administration of steroid hormones, can alter the circadian system in mammals, and suggest that this integrative approach to the study of photoperiodism may prove to be extremely rewarding in our attempts to determine how the length of the day alters reproductive function in mammals.